The invention relates to a method for treating bamboo canes to prevent them from cracking.
Because of its high elasticity and stability, bamboo is used as a constructional element in a variety of ways, predominantly in Asia, from the erection of scaffolding of church-tower height through its use as a material for seating furniture and crockery.
In Europe, bamboo is also used predominantly for producing seating furniture, bed frames and the like.
The fundamental problem with bamboo for these applications is that the moisture content possessed by the bamboo cane during its growth, namely approximately 55-110% by weight, is reduced to that of the air humidity of the surroundings discontinuation of the capillary water supply from the roots. Although in Asia, because the air humidity is usually very high, this difference only leads to cracking of the bamboo cane in isolated cases, when transported to Europe or North America, and especially when used in centrally heated or air conditioned rooms, such canes crack almost regularly because of the very high humidity difference.
Although cracking also adversely affects the stability of the bamboo cane, it is not this that is the decisive disadvantage but rather the impaired optical effect when the bamboo cane has been used as a construction material for relatively high-quality furniture.
Since the bamboo cane has a closed, round cross-section and is subdivided in the longitudinal direction at irregular intervals by transverse walls in the manner of bulkheads, the methods for crack prevention known from the treatment of solid wood processed to form boards (laminated gluing of the solid wood planes with grain directions at angles to one another) cannot be used for crack prevention, since, on the one hand, seen from a purely biological point of view, bamboo is not a type of wood but a giant grass and, even apart from this, there are almost contradictory characteristics from a physical point of view.
Wood has conductive cells for liquid transport predominantly in the outer regions directly under the bark, and a tree trunk therefore has the highest moisture content in its outer region and the lowest moisture content in its core region, the situation is exactly the opposite in the case of bamboo.
In the outer third of the cane there are located the supporting cells which provide the bamboo with its strength, whereas the conducting cells and storage cells for the water to be absorbed together with the nutrients contained therein follow towards the inside. As a consequence, the two substances behave completely differently; in addition, because of its hollow structure with the transverse walls present in the axial direction, the physical behavior of bamboo is also completely different.
Further, the moisture content of bamboo canes, when cut, is not only fundamentally higher than in the case of wood but it also fluctuates much more greatly in dependence on the location, current climate and, considered in the cross-sectional direction of the bamboo cane as well as in the axial longitudinal direction, on the size.
One object of the present invention is therefore to provide a method with which bamboo canes, having two and more transverse walls, can be treated in such a manner that cracking of the bamboo cane after its processing is prevented.
According to the method of the present invention, the mechanical stresses are reduced which normally cause the cracking. These stresses are caused on the one hand by the closed, round cross-sectional contour of the bamboo as a result of which tensile stresses inevitably occur in the outer layer with respect to the inner layer even with uniform percentage shrinkage of the bamboo. This is further reinforced by the internal structure of the bamboo which has more supporting cells in the outer third and more tubular water-transport cells and storage cells in the inner third.
A further difficulty influencing the stresses in the bamboo is posed by the internal transverse walls which are present at irregular intervals and are also manifested by a thickened bead surrounding on the outside of the bamboo.
Since, depending on the growth, the thickness of the wall compared with the diameter of the cane, the tonicity of the cane, the internal structure of the bamboo and its initial moisture content are also different for virtually each individual case, the types of influencing also have to be individually variable.